It is the FTP user/pwd Dreamhost gave you when you signed up. It is not the panel login user/pwd.
A few months ago Dreamhost had some problems and out of safety locked ALL FTP user/pwd's.
You have gotten an email about that and it was in the Dreamhoststatus.com and on top the the panel.
If you have not changed you FTP after that, just login to the panel, go to Users > Manage Users > Edit the user (you).
Change the password and save. Then in FileZilla also change the password.